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Tag Archives: Early Jurassic

Global paleogeographic reconstruction of the Earth in the late Jurassic period 150 Ma. Credit: Dr Ron Blakey

The transition from Triassic to Early Jurassic is marked by a major biotic crisis in the marine and terrestrial realms. In the oceans, this event eliminated conodonts and nearly annihilated corals, ammonites, brachiopods and bivalves. In land, most mammal-like reptiles and large amphibians disappeared, as well as early dinosaur groups. During the Jurassic (201-145 mya) the breakup of the supercontinent Pangaea continued and accelerated with the opening of the North Atlantic by the rifting of Africa and North America, giving rise to the supercontinents of Laurasia and Gondwana. The sea level rise flooded continental areas around Pangaea, forming huge epicontinental seas, especially in northern Africa and eastern Laurasia (modern China). The world was predominantly warm with at least four times the present level of atmospheric CO2. The period is also characterized by the explosive adaptive radiation of dinosaurs and the diversification of the cycads.

The Early Jurassic climate was characterized by a global warming, with average summer temperatures that exceeded 35°C in low-latitude regions of western Pangaea where eolian sandstones testify to the presence of vast deserts (Holz, 2015). The early Toarcian Oceanic Anoxic Event (T-OAE; ∼183 mya) is considered as one of the most severe of the Mesozoic era. It’s associated with a major negative carbon isotope excursion, mass extinction, marine transgression and global warming (Huang, 2014, Ullmann et al., 2014). The T-OAE has been extensively studied in the past three decades although there is no consensus about the causes or triggering mechanisms behind this event.

Painting of a late Jurassic Scene on one of the large island in the Lower Saxony basin in northern Germany (From Wikimedia Commons)

After the extinction of many carnivorous crurotarsan lineages (phytosaurs, ornithosuchids, rauisuchians) at the Triassic–Jurassic boundary, theropod dinosaurs increased their diversity and exhibit a greater range of morphological disparity. Sauropodomorphs also achieved a worldwide distribution and become more graviportal and increased their body size. The presence of early armored dinosaurs (thyreophorans) in North America, Asia, and Europe, but their absent from the southern African record, suggests some degree of provinciality in early ornithischian faunas (Brusatte et al., 2010).

By the Mid-Jurassic, Gondwana started to break up in different blocks: Antarctica, Madagascar, India, and Australia in the east, and Africa and South America in the west, with relatively warm sea-surface conditions (26–30◦C) from Mid-Jurassic (∼160Ma) to the Early Cretaceous (∼115Ma) in the Southern Ocean. There was a drastic climatic decline during the Late Callovian. This decline in temperature lasted about 2.6My and is know as the “Callovian Ice Age”. It has been interpreted in terms of an inverse greenhouse effect, triggered by drawdown of CO2 consequent upon excess carbón burial (Dromart et al, 2003). The Puchezh-Katunki impact crater in Russia is prior to the Callovian extinction event and is not considered as a factor for this biotic extinction event.

During the Late Jurassic, North America completed its separation from Gondwana, and Gondwana was split into a northern and southern continent by the rift system opening the proto-Indian Ocean. The geological and geochemical record suggest that low-latitude environments were arid and tropical ever-wet conditions were absent. Maximum plant diversity was concentrated at midlatitudes, whith forests dominated by a mixture of conifers, cycadophytes, pteridosperms, ferns, and sphenophytes.

Holz, M., Mesozoic paleogeography and paleoclimates – a discussion of the diverse greenhouse and hothouse conditions of an alien world, Journal of South American Earth Sciences (2015), doi: 10.1016/j.jsames.2015.01.001

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In Earth history there have been relatively brief intervals when a very significant expansion of low-oxygen regions occurred throughout the world’s oceans. In mid-1970s the discovery of black shales at many drill sites from the Atlantic, Indian, and the Pacific Ocean led to the recognition of widespread anoxic conditions in the global ocean spanning limited stratigraphic horizons. In 1976, Schlanger and Jenkyns termed these widespread depositional black shale intervals “Oceanic Anoxic Events” (Takashima et al, 2006). This was one of the greatest achievement of the DSDP (Deep Sea Drilling Project).

The Toarcian OAE, Weissert OAE, OAE 1a, and OAE 2 are global-scale anoxic events associated with prominent positive excursions of δ13C and worldwide distribution of black shales. Two models have been proposed to explain it: the stagnant ocean model (STO model) and the expanded oxygen-minimum layer model (OMZ model). Deep-water warming may have also contributed to a decrease in oxygen solubility in the deep ocean and may have triggered the dissociation of large volumes of methane hydrate buried in sediments of the continental margins.

In the Jurassic and Cretaceous oceans, the calcareous nannoplankton was the most efficient rock-forming group, for that reason the characterization of calcareous nannofloras in OAE intervals are used to improve our understanding of the marine ecosystem and biological processes such as photosynthesis (biological pump) and biomineralisation (carbonate pump) that affect the organic and inorganic carbon cycle, as well as adsorption of atmospheric CO2 in the oceans (Erba, 2013). Calcareous nannoplankton represent a major component of oceanic phytoplankton, ranging in size from 0.25 to 30 μm. The first records are from the Late Triassic. Their calcareous skeletons can be found in fine-grained pelagic sediments in high concentrations and the biomineralization of coccoliths is a globally significant rock-forming process.

The early Toarcian Oceanic Anoxic Event (T-OAE; ∼183 mya) in the Jurassic Period is considered as one of the most severe of the Mesozoic era. It’s associated with a major negative carbon isotope excursion, mass extinction, marine transgression and global warming (Huang, 2014, Ullmann et al., 2014). The T-OAE has been extensively studied in the past three decades although there is no general consensus about the causes or triggering mechanisms behind this event. During the peak of the perturbation corresponding to this event, calcareous nannofossils collapsed.

Schizosphaerella punctulata (adapted from Clémence, 2014)

Schizosphaerella is a nannofossil of uncertain biological affinities with a large globular test with two interlocking sub-hemispherical valves formed from a geometric arrangement of equidimensional crystallites with an average value of 10.5 μm in the major axis. During the Early Jurassic, suffered a major drop in abundance, and a reduction in size. The average values drastically decrease down to 8.3 μm around the interval corresponding to the T-OAE. This event is know as ‘Schizosphaerellid crisis’, ‘calcareous nannofossil crisis’ or ‘disappearance event’ (Erba 2004, Clémence, 2014). Four main hypotheses have been proposed to account for the nannoplankton biocalcification crisis through the early Toarcian: (1) a strong stratification of the water column and the development of an oxygen-minimum zone; (2) the discharge of low salinity arctic waters through the Laurasian seaway; (3) high values in atmospheric pCO2; and (4) a rapid warming (Clémence, 2014).

Results from the Paris Bassin as in other localities indicates that the increasing greenhouse conditions may have caused acidification in the oceans, hampering carbonate bio-mineralisation, and provoking a dramatical loss in the CO2 storage capacity of the oceans. The CO2 induced changes in seawater chemistry likely affected the calcification potential of both neritic and pelagic systems, as evidenced by drops of platform-derived carbonate accumulation and drastic reductions in size of the main carbonate producer Schizosphaerella.

The better understanding of the Mesozoic ocean-climate system and the formation of OAEs would help us to predict environmental and biotic changes in a future greenhouse world.